ADHD is a common childhood disorder, affecting roughly 7% of school-aged children. Children diagnosed with ADHD are at risk for academic underachievement, peer rejection, elevated family discord, serious conduct problems, and substance abuse. Although ADHD is one of the most studied childhood disorders, research on its latent structure is in its infancy. One primary question to be explored regarding the latent structure is whether ADHD exists on a continuum or whether it is a unique/discrete condition The long-range goal is to refine the characterization of the phenotypal nature of ADHD. The objective of this application is to determine the latent structure of ADHD using indicators drawn from the NICHD Study of Early Child Care and Youth Development. These indicators will be analyzed using both Paul Meehl's taxometric procedures and factor mixture modeling (FMM). The taxometric analyses will yield dimensional or taxonic (i.e., categorical) results, whereas the FMM analyses can also model various combined structures (e.g., dimensions nested within categories, qualtitatively distinct subtypes). The first specific aim of the study will be to determine the latent structure of ADHD through taxometric and FMM analyses. The second specific aim will be to examine the relations between ADHD and such associated variables as conduct problems, reading difficulties, negative affect, and child-rearing environment, which will help to establish concurrent validity for the latent structure of ADHD. The central hypothesis is that the latent structure of ADHD is primarily unidimensional (i.e., exists on a continuum with quantifiable differences between individuals in the population). Ultimately, determining the latent structure of ADHD will (a) help to refine the diagnostic criteria for ADHD, (b) aid with the development of more sensitive ADHD assessment instruments, (c) have important implications for understanding the etiology of ADHD, and (d) contribute to the development of optimal treatments for children with ADHD. ADHD is a common childhood disorder that is associated with a variety of negative outcomes including academic underacheivement and substance abuse. The proposed study will help determine whether ADHD exists on a continuum or is a discrete disorder, and these findings will have implications for the understanding, diagnosis, and treatment of this condition. [unreadable] [unreadable] [unreadable] [unreadable]